Recently studied technology for moving droplets in one direction, that started from the biomimicry of butterfly wings, spider webs, cactus spines, etc., is technology that has attracted attention in water harvesting, steam condensate drainage in a heat exchanger or the like, or the microfluidics industry.
However, up to now, to move droplets in one direction, a nano or micro-scale structure for helping droplets to move in one direction was manufactured on the surface of a material and tilted and the droplets rolled down in one direction. Alternatively, droplets were able to roll down in one direction by applying an external force exerted by a magnetic field, vibration, or the like.
In Korean Unexamined Patent Application Publication No. 10-2010-0011213 (Feb. 3, 2010), a method for manufacturing a material having a super hydrophobic surface and a super hydrophobic material prepared thereby are disclosed, but a structure capable of moving droplets in on direction without an external force while the surface of the structure is barely tilted has not been disclosed.
Meanwhile, research on super water-repellent technology that started from the biomimicry of butterfly wings, spider webs, cactus spines, etc. has been applied to various materials such as polymers as well as a metal surface in water harvesting or kitchen appliances requiring steam condensate drainage and self-cleaning, and has attracted attention in the microfluidics, aviation and automobile industries, etc.
Until now, to realize the super water-repellency of a polymer material, a water-repellent coating has been used on the polymer, or a nano/microstructure has been directly etched or deposited on the polymer.
However, such methods had difficulty in commercialization due to difficulty in large-scale implementation, a high production cost, etc. Such problems became larger in super water-repellent applications (unidirectional steam condensate drainage, etc.) where regular arrangement of the structures is essential.
On the other hand, the heat transfer performance of the heat exchanger may depend greatly on the wettability of a heat transfer surface with respect to water.
For example, condensation and frosting phenomena occur on the heat transfer surface of the heat exchanger which is applied to a freezer, an air conditioner, a heat pump, etc. To minimize the reduction in efficiency due to the above phenomena, research has been actively carried out to enhance the efficiency of the heat exchanger through the induction of dropwise condensation or the increase in frosting delay, which is caused by water repellency or super water-repellency on the heat transfer surface, which is achieved by manufacturing the nano/micro structure on the heat transfer surface of the heat exchanger through etching, and performing fluoro- or silane-based water repellent or super water-repellent coating by chemical vapor deposition, electrochemical deposition or dip coating.
However, previous research was mostly directed to the dropwise condensation and frosting delay effects achieved by the increase in a contact angle, and research on steam condensate drainage which is important for the enhancement of heat exchanger efficiency is rarely carried out. In addition, since it is difficult to maximize a contact angle for increasing the above-mentioned effects only with a coating material, it is necessary to additionally form a nano/micro structure on the heat transfer surface. However, since the previous research has a limitation in that a process for manufacturing a nano/micro structure cannot be directly applied to a heat exchanger having a complicated shape, which has been already manufactured, the nano/micro structure was manufactured on each part constituting the heat exchanger, and then a water repellent or super water-repellent heat exchanger was manufactured eventually by assembling the individual parts. For example, in Japanese Unexamined Patent Application, First Publication No. H6-307793 (Nov. 1, 1994), submicron-scale micro-irregularities were previously formed on the heat transfer surface facing air, and a nanometer-scale thin film of a branched fluorocarbon-based monolayer was formed on the irregularities for water repellent treatment.
As described above, the previous research has several limitations.
First, even when the dropwise condensation and the frosting delay occurs due to the increase in a contact angle of the heat transfer surface of the heat exchanger, a steam condensate generated on the heat transfer surface was difficult to drain during frosting or defrosting due to a phenomenon in which condensed liquid droplets are attached to the heat transfer surface. Therefore, as frosting/defrosting cycles were repeated, heat transfer performance decreased, and thus a pre-existing heat exchanger was difficult to be practically applied to the heat exchanger industry.
Second, water repellency or super water-repellency is caused by low surface energy and the formation of the nano/micro structure, but the water repellent coating and the nano/micro structure, which can be formed on parts of the heat exchanger to obtain the low surface energy, inherently have low resistance to heat and low mechanical strength. Therefore, due to force or heat applied during the assembly of the individual parts of the heat exchanger, a water repellent coating or a nano/micro structure formed on each part may be damaged, and therefore water repellency or super water-repellency may be considerably degraded.
Third, since the parts of the heat exchanger had to be assembled after water repellency or super water-repellency was realized on each part of the heat exchanger, the process became complicated, and thus much money and time were consumed.